Patente US4781837 - Method of performing osmetic distillation

U.S. Patent Nov. 1,1988 Sheet 4 of 6 4,781,837

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the two different liquids due to the difference in osmotic

METHOD OF PERFORMING OSMETIC pressure between the two liquids.

DISTILLATION It is yet a further object of this invention to provide a

method of, and apparatus for, the concentration of a

TECHNICAL FIELD 5 dilute solution by an osmotic distillation process by the

~ . . t- i * * tu„ „„ *. *; „f transfer of solvent from the dilute solution in a vapour

The present mvention relates to the concentration of ^ ^ it_ , , , , , . . „ ., . y

i- -j u *-r • *u J-cc r _ *• .. state through a hydrophobic (non-wettable) porous

a liquid by utilising the difference of osmotic pressure . ^ K * , ru- u

, , / v -j J * * ui .i » barner into a concentrated solution of higher osmotic

between two liquids, and to apparatus suitable there- e

- ^ rr pressure.

10 It is yet another object of this invention to provide a

BACKGROUND ART process for the fractionation of, or recovery of a solvent

„ .. A. - . . . from, a solution by a combination of osmotic distillation

The application of reverse osmosis to the concentra- ... Q^ ses

tion of solutions, for example dilute food products, and other g rf ... ^ fee

to the production of potable water or water of reduced , 5 ... from ^ fo,lo^ disdosure of the ^^J,

salts content from seawater has m many cases resulted According t0 one ^ect of the resent inve„ti0n

in significant cost benefits when compared with the ... u ^ a s ... orHdiffusion barrier

conventional evaporation and distillation processes re- for ^ concentration of a dilute solution of low osmotic

spectively Nevertheless, there is scope for a further fe fe ... distillation comprising an hydro.

substantial cost reduction by eliminating the fouling 2Q hobic g of s matrix wherein solvent

problem associated with hydrophihc reverse osmosis from ^ dUute ... is transferred th h the bar.

membranes The wettability of these membranes pro- rier from Qne side tQ thfi ite side in a r state

vides for close contact with an adherence of foulants undef thg -m{luence of an osmotic sure gradient.

such as organic colourants, various inorganic salts, and According to the invention a method of making such

suspended and colloidal matter. Fouling has in some 25 a diffusion barrier comprises chemical treatment of the

cases been alleviated by precipitation and filtration pre- porous matrix with an agent tQ mcIease the hydropho.

treatment processes. However, this adds substantially to bidty or the non.wettable properties thereof.

the cost of the overall process, thus reducmg the attrac- According to another aspect of the present invention

tiveness of the reverse osmosis technique. there is provided a method of osmotic distillation for

Hydrophobic porous barriers (e.g. diffusion barriers, 30 the concentration of a first iiquid of relatively low ossuch as hydrophobic porous or microporous mem- motic pressure by circulating said liquid on one side of branes) of various pore dimensions have been devel- a hydrophobic porous barrier or membrane whilst sioped for various applications, e.g. for membrane distil- multaneously circulating a second liquid of relatively lation and artificial oxygenation. high osmotic pressure on the opposite side of the barrier

Hydrophobic membranes are generally made from an 35 whereby solvent from said first liquid is transferred

hydrophobic polymer, e.g. polytetrafluoroethylene across the barrier in the vapour state to the second

(TEFLON) or polypropylene, such as membranes pro- liquid reSulting in the concentration of the first liquid,

duced under the name CELGARD. According to a further aspect of the invention there is

Hydrophobic membranes can also be made from provided apparatus for the fractionation of a primary

hydrophillic polymers which have been specially 40 liquid of low osmotic concentration and recovery of the

treated to have reticulation of fluorocarbon radicals at solvent therefrom, comprising:

the surface of the polymer. (j) a p0rous barrier for osmotic distillation comprising

Hydrophobic membranes are widely used in the med- an hydrophobic porous matrix wherein solvent

ical industry for artificial oxygenation of blood. Such from the primary liquid on one side of the porous

artificial oxygenation units are discarded after use, and 45 barrier is transferred through the porous barrier in

are readily available from hospitals after use. a vapour state to a second liquid of higher osmotic

Membrane distillation processes are known which concentration on the opposite side of the porous

rely on the temperature gradient between two solutions. barrier under this influence of an osmotic pressure

However, an osmotic distillation process which relies gradient; and

on the difference in osmotic pressure between two solu- 50 (ii) a diffusion barrier for reverse osmosis whereby

tions has not previously been described. the diluted second liquid from the previous step

DISCLOSURE OF THE INVENTION may subsequently be split up to reconstitute and

recycle the second liquid and recover the solvent

It is an object of this invention to provide a method of transferred from the primary liquid,

increasing the hydrophobicity of a porous barrier or 55 According to yet another aspect of the invention

membrane and to provide membranes prepared by said there is provided a method for recovery of a solvent

method. (e.g. potable water) from a primary liquid of relatively

It is a further object of this invention to provide a low osmotic concentration (e.g. seawater), comprising:

method of, and apparatus for, the concentration of a (i) subjecting the primary liquid to osmotic distilla

liquid by utilising the difference of osmotic pressure 60 tion with a porous barrier which separates the

(i.e., osmotic pressure gradient) between two liquids. primary liquid from a second liquid of higher os

It is another object of the present invention to recycle motic concentration characterized in that said po

such membrane oxygenation units for use in a mem- rous barrier comprises an hydrophobic porous

brane separation process using as the driving force the matrix and wherein an osmotic pressure gradient

difference of the osmotic pressure between two liquids 65 exists between said primary and second liquids

to achieve a concentration of the liquid having the whereby solvent from the primary liquid on one

lower osmotic pressure, or to recover part of the energy side of the diffusion barrier is transferred through

available between the two different potential energies of the porous barrier in a vapour state to the second

liquid on the opposite side of the porous barrier; and

(ii) further subjecting the diluted second liquid from the previous step to reverse osmosis by reverse osmosis means whereby the solvent (e.g. potable 5 water) is recovered. Osmotic distillation is a process by which dilute solutions such as seawater, fruit juices, milk and coffee may be concentrated by transfer of water in the vapour state and under the influence of an osmotic pressure gradient 10 through a hydrophobic (non-wettable), non-fouling porous barrier or membrane into seawater or a concentrated brine (e.g. magnesium sulphate, MgSCH solution) which is free of fouling material. The latter solution may then be treated by reverse osmosis for reconcentra- 15 tion and recycling to the osmotic distillation unit and, in the case of seawater treatment, for the recovery of potable water. In this way the expected life of reverse osmosis membranes can be extended significantly.

BRIEF DESCRIPTION OF THE DRAWINGS 20

The invention will be further described with reference to the drawings, in which:

FIG. 1 schematically illustrates the concentration of fruit juice against seawater or concentrated brine by 25 osmotic distillation.

FIG. 2 schematically illustrates apparatus according to the invention wherein a osmotic distillation tube is coupled to a reverse osmosis unit for use in e.g. desalination—i.e., the production of potable wter of high purity 30 from brackish water or seawater.

FIG. 3 is an enlarged view of the membrane wall of the osmotic distillation apparatus of FIG. 2, schematically illustrating that one of the main driving forces for the transfer of water from the seawater across the mem- 35 brane wall and into the MgS04 solution is a vapour pressure gradient generated by an osmotic pressure gradient.

FIG. 4 is a graph illustrating the reverse osmosis flux versus MgS04 concentration at an operating pressure of 40 4130 kPa (40.8 atm) of apparatus of the type illustrated in FIG. 2.

FIG. 5 is a graph illustrating the osmotic distillation flux versus MgS04 concentration for a distilled water/MgS04 system (at 30 kPa water back pressure). 45

FIG. 6 is a graph illustrating the osmotic distillation flux versus the temperature difference between the temperature of the inlet distilled water and the temperature of the inlet MgS04 solution (at 30 kPa water back pressure). 50

FIG. 7 is a graph illustrating the osmotic distillation flux versus MgS04 concentration for a seawater/MgS04 system (at 30 kPa seawater back pressure).

FIG. 8 is a graph illustrating the osmotic distillation flux versus the temperature difference between the tern- 55 perature of the inlet seawater and the temperature of the inlet MgS04 solution (at 30 kPa seawater back pressure).

FIG. 9 is a graph illustrating the osmotic distillation flux versus seawater flow rate (14% MgS04, 40 kPa 60 seawater back pressure, 0.711/min MgS04).

FIG. 10 is a graph illustrating the osmotic distillation flux versus MgS04 solution flow rate (14% MgS04, 40 kPa seawater back pressure, 1.36 1/min seawater flow rate). 65

FIG. 11 is a graph illustrating the relationship between the osmotic distillation flux and MgS04 concentration under preferred conditions for this example.

BEST MODE OF CARRYING OUT THE
INVENTION

In the drawings, FIG. 1 schematically illustrates the concentration of orange juice in a hollow fibre osmotic distillation unit wherein fresh orange juice at a concentration of 12° Brix is passed through the centre lumens of the bundle of hollow fibres in the osmotic distillation unit and seawater is passed countercurrent in the jacketed space of the osmotic distillation unit surrounding the bundle of hollow fibres. The osmotic-distillation unit comprises a cylindrical polycarbonate outer shell (diameter 100 mm, length 300 mm) with conical entry (top) and exit (bottom) chambers for the orange juice being concentrated. The entry and exit chambers are linked by ca. 62,000 hydrophobic polypropylene hollow fibre membranes (inside diameter 200 /xm, wall thickness 25 u,m, effective length 140 mm, average pore diameter 700 A, porosity 50 percent, total area 5.4 m2) which are sealed together at both ends in a polyurethane resin (potting compound) so that they are contained in a jacket with entry and exit ports. Filtered seawater containing two suspended or colloidal material is pumped through this jacket approximately countercurrent to the internal flow.

The pure orange juice pumped through the osmotic distillation tube under low pressure becomes concentrated as it passes through the hollow fibres due to a transfer of water across the fibre walls into the seawater. The driving force for this transfer is the combined effect of a vapour pressure (or osmotic) gradient and a temperature gradient, brought about by an elevation of the vapour pressure of the orange juice by gentle heating before it enters the tube, and a depressed seawater vapour pressure by virtue of its relatively high osmotic pressure. The hydrophobicity of the membrane prevents transfer of liquid water and the fouling problems associated with hydrophilic membranes.

FIG. 2 shows the coupling of a osmotic distillation tube and reverse osmosis unit suitable for desalination of seawater or brackish water.

A suitable osmotic distillation tube comprises a cylindrical polycarbonate outer shell (diameter 100 mm, length 300 mm) with conical entry (top) and exit (bottom) chambers for the solution being concentrated. The entry and exit chambers are linked by ca. 62,000 hydrophobic polypropylene hollow fibre membranes (inside diameter 200 jxm, wall thickness 25 fim, effective length 140 mm, average pore diameter 700 A, porosity 50 percent, total area 5.4 m2) which are sealed together at both ends in a polyurethane resin (potting compound) so that they are contained in a jacket with entry and exit ports. A concentrated brine solution (e.g. magnesium sulphate, MgS04) containing no suspended or colloidal material is pumped through this jacket approximately countercurrent to the internal flow.

Normal (dilute) seawater pumped through the osmotic distillation tube under low pressure becomes concentrated as it passes through the hollow fibres due to a transfer of water across the fibre walls into the MgS04 solution. The driving force for this transfer is a vapour pressure gradient, brought about by an elevation of the vapour pressure of the dilute solution by heating before it enters the tube, and a depressed MgSo4 solution vapour pressure by virtue of its high osmotic pressure. The hydrophobicity of the membrane prevents transfer of liquid water and the fouling problems associated with hydrophilic membranes.

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Method of performing osmetic distillation